Gas turbine manufacturers are regularly involved in research and engineering programs to produce new gas turbines that will operate at high efficiency and produce less undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons.
The oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen (NOx) is a function of temperature. If the temperature of the combustion chamber hot gas is controlled to a sufficiently low level, thermal NOx will be reduced.
One method of controlling the temperature of the reaction zone of a combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where production of thermal NOx is reduced.
There are several problems associated with dry low emissions combustors operating with lean premixing of fuel and air. Flammable mixtures of fuel and air exist within a premixing section of the combustor, which is external to the reaction zone of the combustor. One problem is a tendency for combustion to occur within the premixing section due to flashback. Flashback occurs when flame propagates from the combustor reaction zone into the premixing section. The flame held inside the wake flows behind the fuel injection columns (jet cross flow) or vane trailing edges.
Another problem is autoignition. Autoignition occurs when the dwell time and temperature for the fuel/air mixture in the premixing section are sufficient for combustion to be initiated without an igniter. The consequences of combustion in the premixing section are degradation of emissions performance and/or overheating and damage to the premixing section, which is typically not designed to withstand the heat of combustion.
Therefore, a problem to be solved is to prevent flashback, flame holding, and/or autoignition resulting in combustion within the premixing section. As flashback, flame holding and/or autoignition concerns rise due to use of more reactive fuels, the need for a reliable flashback detection method increases.